The File only contains the .DDS files.
If you need to know how to add .DDS files to your GFX.mix file, see Thimoty Decrans page here.
OR
Watch the video:

MixFile Remixer available here:http://www.racedepartment.com/forum/threads/mixfile-remixer.71456/
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Also, please notify me if any of the links do not work so that I can fix them ASAP.